1. Field of the Invention:
The present invention relates to novel protonic, conductive, electrolytic materials. It more particularly relates to the production of electrochromic systems mainly operating in light transmission, such as glazings for buildings or cars.
2. Discussion of the Background:
EP-A-253 713 discloses an electrochromic system operating by the insertion/deinsertion of protons in a layer of an oxide of a transition metal such as tungsten trioxide in contact with a solid electrolyte such as a protonic, conductive, electrolytic, macromolecular material formed by a phosphoric acid-polyoxyethylene (H.sub.3 PO.sub.4 -POE) complex. This material must be prepared under anhydrous conditions, taking particular care with the choice of solvents, in order to avoid a rapid deterioration of the other layers of the system associated with the electrolyte and in particular electrochromic material layers or electronically conductive layers or electrodes, between which is applied the potential difference necessary for coloration or decolorization.
Moreover, in order to prevent bubbling, which, after a few hours of operation totally degrades the optical quality of the system, EP-A-477 065 recommends a thorough degassing of the electrolyte, in vacuo and preferably at a high temperature, not only to avoid the trapping of air during the assembly of the glazing, but also so as to evacuate the air, or any solvent traces, trapped in the first polymer preparation phases.
This treatment is particularly effective in that it eliminates the bubbles which normally appear after a few dozen hours of operation. It also reduces the haze phenomenon, or in other words the proportion of the light diffused through the glazing, which when it becomes excessive no longer permits correct vision through the glazing. However, the degassing or more specifically the heating operation performed during the degassing (typically 3 hours at a temperature of 100.degree./120.degree. C.) does not permit the elimination and even increases the yellowing of the polymer and the purity in the yellow, the latter essentially representing the perception of the color by the eye. This coloration of the glazing, in the decolorized state, is undesirable and in any case yellow is not very highly esteemed for building glazings.
In addition, this hot degassing treatment acts in precisely the same way as a thermal test and causes the deterioration of the polymer as when exposed to high temperatures. In this regard, it should be noted that an electrochromic system used as an external glazing can reach or exceed temperatures of about 80.degree. C. under the combined effects of solar radiation and the black body effect when the glazing is in the colored state. Independently of any hot degassing treatment, there is consequently a problem of aging by yellowing of the polymer at temperatures of this level.
Analysis of the problem reveals that this yellowing is probably due to the formation of free radicals, which is aided by an acid medium. Although it is known to delay the formation of free radicals and/or block the action of the free radicals formed by means of an antioxidant, to date it was not known what antioxidants, if any, would be compatible with an acid medium and the synthesis solvents and would not affect the optical properties of the polymer, where it is desirable to have a maximum absence of coloration, a high light transmission and a high quality of transmission, i.e., the absence of any significant light diffusion.
Another problem encountered during the thermal tests is that of a drastic decrease in the viscosity of the material. Thus, after about 20 hours at 100.degree. C. the flow of the material is already significant, and after about 100 hours the viscosity is so reduced as to be completely incompatible with a vertical position of the glazings, which can admittedly be accepted, e.g., for car sunroofs, but not for building glazings. Thus, during the assembly of the electrochromic system by a procedure similar to that currently used for the preparation of laminated glazings, the polymer is heated and an excessive flow is a handicap. It has also been found that this high temperature flow phenomenon even occurs in the absence of a hot drying treatment of the polymer, although such hot drying treatments tend to accentuate the problem.
Thus, there remains a need for protonic, conductive electrolytic materials which exhibit a reduced tendency to undergo yellowing. There also remains a need for protonic, conductive electrolytic materials which maintain a high viscosity even at high temperatures.